Environmental concerns and decreasing amounts of fossil fuels, especially oil, requires the development of renewable resources for liquid fuels, which are critical for transportation and industrial sectors. Solar energy and solar fuels have been touted as the ultimate, indefatigable sources of energy, but the storage of sunlight and its use in a practical way remains a major challenge of modern times.
Biomass has potential as a renewable resource and solar-based fuel where the conversion of water and carbon dioxide to glucose and then to other organic materials is achieved by photosynthesis followed by further biosynthetic pathways. Thus, the storage of solar energy in terrestrial biomass takes advantage of natural photosynthetic pathways and further biosynthetic reactions using CO2 and H2O as carbon and hydrogen atom sources. Terrestrial plants contain hemicellulose, ˜(C5H10O5)n, and cellulose, (C6H12O6)n, that can be re-processed to obtain a convenient hydrocarbon fuel. Typically, biomass contains 35-50% cellulose, 20-35% hemicellulose, and 10-25% lignin. Biorefining of biomass is an emerging field, and processes including catalytic hydrolysis, solvolysis, liquefaction, pyrolysis, gasification, hydrogenolysis and hydrogenation are all being considered.(1,2) The transformation of cellulose to D-glucose, its sole component, and then to ethanol to be used as a biofuel by fermentation is perhaps the most developed technology, however the failure of simple acids to hydrolyze cellulose selectively (3) remains a problem that requires the use of more expensive cellusomes.(4) The hydrolysis/fermentation approach also has the disadvantage that hemicellulose and its major hydrolysis product, D-xylose, still cannot be used to form ethanol. On the other hand, cellulose/hemicellulose hydrolysis has been suggested as a route to chemicals such as sugars, formic acid, levulinic acid and hydroxymethylfurfuraldehyde among others.(5,6) Other efforts have been made to convert cellulose or cellulose derived materials to hydrocarbons,(7-9), hydrogen,(10,11), and furans.(12) Despite the aforementioned advances, reaction selectivity is typically low and process schemes are quite complicated.
Synthesis gas (also known as Syngas), is a fuel gas mixture consisting primarily of hydrogen (H2), carbon monoxide (CO), and often some carbon dioxide (CO2). Synthesis gas is a known industrial commodity that is used an intermediary building block for the production of various fuels such as synthetic natural gas, ammonia, methanol, and synthetic petroleum fuel. For example, hydrocarbons can be prepared from synthesis gas via the Fischer-Tropsch process (13), or methanol can be synthesized both as a fuel and fuel precursor. (14) Synthesis gas may also be used as a direct fuel source. In a purified state, the hydrogen component of synthesis gas can also be used to directly power hydrogen fuel cells for electricity generation and fuel cell electric vehicle propulsion. Hydrogen can also be used to prepare ammonia by the Haber-Bosch process.
It has previously been shown by Khenkin et al., that a phosphovanadomolybdic acid such as the H5PV2Mo10O40 polyoxometalate catalyzes the carbon-carbon bond cleavage of vicinal diols and primary alcohols.(15) In this electron transfer-oxygen transfer type reaction, oxygen atoms from the polyoxometalate are inserted into the carbon-carbon bond and the hydrogen atoms released are retained on the polyoxometalate as protons and electrons. For example, the initial product of ethylene glycol oxidation was formaldehyde and 1-butanol oxidation initially yielded formaldehyde and propionaldehyde. Wasserscheid et al. showed that H5PV2Mo10O40 can convert carbohydrates such as hemicellulose in water under rather high O2 pressures (30 bar) to formic acid in a 50% yield with co-formation of 50% CO2, but cellulose reacted in only low yields.(16,17) Vanadium oxide was similarly reactive in this reaction.(18) Analysis of the results shows that the maximum yield of formic acid is around 50% for hemicellulose and around only 7% for cellulose.(16-18) It was suggested (16-18) that formic acid could be further transformed to H2 and CO2, typically over a noble metal catalyst as previously described.(19,20) Alternatively, acid catalyzed dehydration of formic acid to CO and H2O can be contemplated.(21) Using D-glucose as a model, this translates into a potential yield, at the observed 50% yield of formic acid, of 3 mol of H2 or CO but not both per mol D-glucose. This is far from the optimum of 6 mol of both CO and H2 per mol D-glucose.
U.S. Patent Application No. US 2013/0245319 discloses a method for catalytically producing formic acid, by reacting an alpha-hydroxyaldehyde, alpha-hydroxycarboxylic acid, a carbohydrate or a glycoside with a polyoxometallate ion catalyst, of the general formula [PMoxVyO40]q−, using water as a solvent.
Weinstock, I. A. et at (26) describes the use of Keggin-type polyoxometalate (POM) salts and oxygen to bleach wood pulp for use in the manufacture of paper.
U.S. Pat. Nos. 5,302,248, 5,549,789 and 5,695,606 describe a method for delignifying/bleaching wood pulp for the manufacture of paper. The bleaching process involves exposing the wood pulp to a polyoxometalate of the formula [V1-MomWnNboTap(TM)qXrOs]x−, to produce water soluble oxidized lignin, which is then oxidatively degraded to CO2 by heating the solution to high temperatures.
Synthesis gas is an important commodity used for formation of fuel and fuel precursors, among other industrial applications. There remains an unmet need for efficient methods for preparing synthesis gas from biomass.